[laurencerowe]

> So the findings here do make sense. For sub 5m cables directly connecting two machines is going to be faster then having some PHY in between that has to resignal. I'm surprised that fiber is only 0.4ns/m worse then their direct copper cables, that is pretty incredible.

Surely resignaling should be the fixed cost they calculate at about 1ns? Why does it also incur a 0.4ns/m cost?

[cenamus]

Light speed is ~3ns per metre, so maybe the lowered speed through the fibre?

Speed of electricity in wire should be pretty close to c (at least the front)

[myself248]

Velocity factor in most cables is between 0.6 and 0.8 of what it is in a vacuum. Depends on the dielectric material and cable construction.

This is why point-to-point microwave links took over the HFT market -- they're covering miles with free space, not fiber.

[jcims]

I always thought it was about reduced path length. Interesting.

[cycomanic]

It's both. Those links try to minimise deviation from the straight link (and invest significant money to get antenna locations to do that), but they also use copper/coax cables for connecting radios as well as hollow core fibre for other connections to the modems.

[laurencerowe]

I misremembered the speed of electrical signal propagation from high school physics. It's around 2/3rds the speed of light in a vacuum not 1/3rd. The speed of light in an optical fibre is also around 2/3rds the speed in a vacuum.

It seems there is quite a wide range for different types of cables so some will be faster and others slower than optical fibre. https://en.wikipedia.org/wiki/Velocity_factor

But the resignalling must surely be unrelated?

[throw0101b]

> Light speed is ~3ns per metre, so maybe the lowered speed through the fibre?

Obligatory Adm. Grace Hopper nanosecond reference:

* https://www.youtube.com/watch?v=si9iqF5uTFk&t=40m10s

[b3orn]

It's c, but not the same c as in air or vacuum. The same applies in optic fibers. They're both around two thirds of the speed of light in vacuum.

[GuB-42]

c is constant, the speed of light is not.

c is the speed of light in a vacuum, but it is not really about light, it is a property of spacetime itself, and light just happens to be carried by a massless particle, which, according to Einstein's equations, make it go at c (when undisturbed by the medium). Gravity also goes at c.

[bigfishrunning]

I've always considered C the speed of light and gravity goes at the speed of light, not that light and gravity both go C, which is a property of spacetime. This is a much simpler mental model, thanks for the simple explanation!

[Sniffnoy]

You can think of c as the conversion rate between space and time; then, light (and anything else without mass, such as gravity or gluons) travels at a speed of 1. Everything else travels at a speed of less than 1.

(Physicists will in fact use the c=1 convention when keeping track of the distinction between distance units and time units is not important. A related convention is hbar=1.)

You can tell that c is fundamental, rather than just a property of light, from how it appears in the equations for Lorentz boosts (length contraction and time dilation).

[Eldt]

I've always thought of c as the speed limit of causality

[Sesse__]

c is the speed of light in vacuum.

EM signals move at about 0,66c in fiber, and about 0,98c in copper.

[BenjiWiebe]

More like 0.6c to 0.75c in Cat6 Ethernet cable.

The insulation slows it down.

[colanderman]

Don't know why you were downvoted, this is true. RF energy is carried primarily (solely?) by the dielectric, not the copper itself, simply by virtue of the fact that this is where the E and M fields (and therefore Poynting vector) are nonzero. It's therefore the velocity factor of the dielectric which is relevant.

[deepsun]

What if I make the copper wire hollow inside, filled with vacuum? Will the signal travel at c?